The Faraday effect can be described as the polarization rotation ("Faraday Rotation") which occurs when linearly polarized light is transmitted through an optical material exposed to a magnetic field aligned parallel to the light's propagation direction. Materials which exhibit the Faraday effect are called magneto-optic materials. Since in many magneto-optic materials the amount of rotation is proportional to the strength of the magnetic field, the magnetic field strength can be determined by measuring the angle of polarization rotation. This concept is the basis of all Faraday effect sensors.
Magneto-optic materials which can be used in Faraday effect sensors can either be diamagnetic, paramagnetic, or ferrimagnetic. Diamagnetic materials have very low magnetic permeability and also low magneto-optic sensitivity (i.e. polarization rotation per unit magnetic field strength). Paramagnetic materials also have low permeability, but are more sensitive than diamagnetic materials. Finally, ferrimagnetic materials, such as the iron garnets, exhibit very high permeability and higher magneto-optic sensitivity than either diamagnetic or paramagnetic materials.
The basis for the present invention is the increased sensitivity which results when a high-permeability magneto-optic material (such as yttrium iron garnet, or YIG) is combined with one or more high-permeability flux concentrators as illustrated in FIG. 2. Unlike previous Faraday effect sensors which have combined magneto-optic materials with flux concentrators, the present invention is intended and designed for the general measurement of uniform magnetic fields.
The idea of combining a conventional magnetic field sensor with a magnetic flux concentrator (i.e. a high-permeability device designed to concentrate magnetic fields) has previously been used to create sensors specifically designed for measuring electric current. High-permeability magneto-optic materials, Hall Probes, and magnetoresistive devices have all been used as the sensing element in these electric,current sensors. Typically, a current-carrying conductor passes through a gapped toroidal-shaped flux concentrator. The magnetic field generated by the current in the conductor is concentrated across the gap in the toroid. The sensing element, located in the gap, typically produces an electrical signal proportional to the magnetic field in the gap. Since the magnetic field in the gap is proportional to the current in the conductor, the signal generated by the sensor is a measure of the electric current.
Below follows a summary of the prior art including reference to the state of the art improvements to sensors based on high-permeability magneto-optic materials. Only U.S. Pat. No. 4,692,703 (1987) to Extance, et al. teaches the use of linear overlapping flux concentrators surrounding a magnetic field sensor. Extance uses a Hall effect sensor as the magnetic field sensing element. His invention could be expected to yield increased sensitivity for measuring uniform magnetic fields including those of the earth. However, the effect of the flux concentrators in his sensor is not optimized because the Hall sensor, located in the overlapping region of the flux concentrators, has low permeability (since it is diamagnetic) and, therefore, breaks the magnetic continuity of the two flux concentrators.
The present invention uses a high-permeability magneto-optic material having a small diameter relative to a surrounding pair of high-permeability flux concentrators. Thus, the magneto-optic material and flux concentrators form a continuous magnetic circuit which results in much more efficient flux concentration than sensors which combine flux concentrators with low-permeability sensors (such as Hall probes). In the preferred embodiment, the cylindrical flux concentrators are tapered towards the high-permeability magneto-optic material to minimize flux leakage (and maximize flux concentration). Finally, the flux concentrators have a hole along their axis to allow the linearly-polarized beam of light to pass directly through both the flux concentrators and the (transmissive) high-permeability magneto-optic material. The diameter of the high-permeability magneto-optic material approximately equals the hole diameter of the flux concentrators, allowing the two ends of the magneto-optic cylinder to be partially inserted into the respective flux concentrator holes.
When a sample of yttrium iron garnet is used as the magneto-optic sensing element, the result is an increase in sensitivity of two hundred times compared to the sensitivity of the yytrium iron garnet alone.